The peripheral nervous system is a collection of nerves that connect the brain and spinal cord to other body parts (peripheral system).

Definitions

Somatic nervous system

Innervate the skin and muscle of voluntary control

Autonomic nervous system

Innervate the visceral organs such as heart, stomach, liver; control is involuntary

Lower motor neuron lesion

Nerve damage to nerve fibers that connect the spinal cord to the peripheral muscles; common symptoms will be muscle weakness, muscle atrophy, fasciculations, hypotonia, and hyporeflexia

Fasciculations

Involuntary muscle twitching that occurs when peripheral nerve is overactive and damaged

Components of peripheral nervous system

There are four (4) major components of the peripheral nervous system– cranial nerves, spinal nerves, peripheral nerves, and the nerves that comprise the autonomic nervous system.

• Cranial nerves
  • 12 pairs of nerves arising from the brainstem responsible for sensory and motor functions of the head and neck
  • List of cranial nerves
    • Olfactory nerve-I:
      • Sensory only
        • Function: smell
    • Optic nerve - II:
      • Sensory only
        • Function: visual acuity
    • Oculomotor nerve- III:
      • Motor only
        • Function: move eye up, down, and in (medially)
    • Trochlear nerve -IV:
      • Motor only
        • Function: move adducted eye down (torsional movement)
    • Trigeminal nerve - V:
      • Sensory and motor
        • Function: sensory -touch, pain and temperature to face; motor - muscles of mastication
    • Abducens nerve - VI:
      • Motor only
        • Function: moves out out (laterally)
    • Facial nerve - VII:
      • Sensory and motor
        • Function: sensory- taste to anterior two-thirds of tongue; motor- facial expressions
    • Vestibulocochlear nerve- VIII:
      • Sensory only
        • Function: balance and hearing acuity
    • Glossopharyngeal nerve - IX:
      • Sensory and motor
        • Function: sensory- taste to posterior one-third of tongue; motor- gag reflex, pharynx and larynx control
    • Vagus nerve- X:
      • Sensory and motor
        • Function: sensory- autonomic nervous system; motor- pharynx and larynx control, assist with digestion, aids in slowing heart rate, and bronchial constriction
    • Accessory nerve- XI:
      • Motor only
        • Function: control trapezius and sternocleidomastoid muscle
    • Hypoglossal nerve - XII:
      • Motor only
        • Function: controls tongue movements

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Spinal nerves

• Thirty-one (31) pairs of nerves exiting the varying vertebral areas- divided into dorsal (sensory) and ventral (motor) roots
  • Each spinal nerve corresponds with a segment of the spinal cord (i.e. 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal)

Structure of peripheral nerves

Consists of the following:

• Nerve fibers
  • Axons send electrical signals from dendrites to cell body
• Connective tissue
  • The endoneurium, perineurium, and epineurium surrounding and binding the nerves providing protection from injury
• Blood vessels
  • Aid in vascularizing nerve
• Myelin
  • Fatty tissue surrounding and protecting the nerve

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Definitions

Axon

Projection that conducts electrical impulses away from the nerve cell body

Dendrite

Receiving connection of the nerves

Nerve fascicle

Bundle of nerve fibers that are targeting the same anatomical location- located inside of dendrite

Perineirium

Innermost layer of connective tissue that surrounds a nerve fascicle

Endoneurium

Layer of connective tissue associated with the peripheral nerve composed of collagen, fibroblasts, macrophages, or mast cells that provinces protection to nerve fiber

Epineurium

Outermost layer of connective tissue that surrounds a nerve fascicle

Autonomic nervous system

• Provides motor innervation to smooth muscle, cardiac muscle, and glands to provide unconscious control
• Further divided into sympathetic, parasympathetic, and enteric divisions- each division works to maintain homeostasis within the body
  • Sympathetic nervous system
    • Works to manage individual’s response to anxiety, stress
  • Parasympathetic
    • Works to manage individual at rest
  • Enteric system
    • Management of gastrointestinal system

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Dermatomes and myotomes

A dermatome is an area on the skin associated with a specific spinal nerve root (31 spinal nerve roots) in which sensory information is carried. A myotome is the link between a group of peripheral muscles and a specific spinal nerve root (31 spinal nerve roots) in which motor information is carried. Dermatomes and myotomes are used to diagnose the potential damage of a spinal nerve root. Review the below dermatome map and myotome chart.

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Dermatome Map

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Myotomes

• Upper extremity
  • C1- cervical rotation
  • C2-C4- shoulder elevation
  • C5- shoulder abduction/elbow flexion
  • C6- wrist extension
  • C7- elbow extension/wrist flexion
  • C8- thumb extension
  • T1- finger adduction
• Lower extremity
  • L1-L2: hip flexion
  • L3: knee extension
  • L4: ankle dorsiflexion
  • L5: great toe extension
  • S1: ankle plantar flexion
  • S2: knee flexion

Spinal reflexes

Spinal reflexes (also called deep tendon reflexes) are automatic, involuntary responses of the motor system in response to sensory stimuli. Spina reflexes occur in the spinal cord and thus allow the body to respond quickly without conscious thought. Review the spinal reflexes below.

• Spinal reflexes
  • C5: biceps reflex
  • C6: brachioradialis reflex
  • C7: triceps reflex
  • L3: patellar reflex
  • S1: achilles reflex

Example of reflexes work in spinal cord below:

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Brachial, lumbar, and sacral plexuses

The brachial, lumbar, and sacral plexuses are a network of nerves that form a connection between the peripheral nerves and central nervous system. The peripheral nerves innervating all muscles are derived from the brachial, lumbar, and sacral plexuses.

The brachial plexus provides innervation to the chest and upper limbs.

The lumbar plexus provides innervation to the abdomen, groin, anterior thigh and leg, and feet.

The sacral plexus provides innervation to the pelvis, buttocks, feet, and genitals.

Brachial plexus

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Muscles Innervated by the Brachial Plexus

Roots (C5–T1)

• Dorsal scapular nerve (C5):
  • Rhomboid major
  • Rhomboid minor
  • Levator scapulae
• Long thoracic nerve (C5–C7):
  • Serratus anterior

Trunks

• Suprascapular nerve (Upper trunk: C5–C6):
  • Supraspinatus
  • Infraspinatus
• Nerve to subclavius (Upper trunk: C5–C6):
  • Subclavius

Divisions

• No direct muscle innervations from divisions.

Lateral Cord

• Lateral pectoral nerve (C5–C7):
  • Pectoralis major (clavicular head)
• Musculocutaneous nerve (C5–C7):
  • Biceps brachii
  • Brachialis
  • Coracobrachialis

Medial Cord

• Medial pectoral nerve (C8–T1):
  • Pectoralis major (sternal head)
  • Pectoralis minor
• Medial cutaneous nerves (arm & forearm):
  • No motor function (sensory only)
• Ulnar nerve (C8–T1):
  • Flexor carpi ulnaris
  • Medial half of flexor digitorum profundus
  • Most intrinsic hand muscles:
    • Hypothenar muscles (abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi)
    • Adductor pollicis
    • 3rd and 4th lumbricals
    • Palmar and dorsal interossei

Posterior Cord

• Upper subscapular nerve (C5–C6):
  • Subscapularis
• Thoracodorsal nerve (C6–C8):
  • Latissimus dorsi
• Lower subscapular nerve (C5–C6):
  • Subscapularis
  • Teres major
• Axillary nerve (C5–C6):
  • Deltoid
  • Teres minor
• Radial nerve (C5–T1):
  • All muscles in the posterior arm and forearm:
    • Triceps brachii
    • Anconeus
    • Brachioradialis
    • Extensor muscles of wrist and fingers (e.g., extensor carpi radialis longus/brevis, extensor digitorum, extensor carpi ulnaris, extensor pollicis muscles)
    • Supinator

Median Nerve (from both Lateral and Medial Cords)

• Most anterior forearm muscles:
  • Pronator teres
  • Flexor carpi radialis
  • Palmaris longus
  • Flexor digitorum superficialis
  • Lateral half of flexor digitorum profundus
  • Flexor pollicis longus
  • Pronator quadratus
• Thenar muscles:
  • Abductor pollicis brevis
  • Flexor pollicis brevis (superficial head)
  • Opponens pollicis
• 1st and 2nd lumbricals

Lumbar plexus

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Muscles Innervated by the Lumbar Plexus (L1–L4)

Iliohypogastric Nerve (L1)

• Internal oblique
• Transversus abdominis

Genitofemoral Nerve (L1–L2)

• Cremaster muscle (in males; part of spermatic cord)

Lateral Femoral Cutaneous Nerve (L2–L3)

• No motor innervation – sensory only

Femoral Nerve (L2–L4)

• Iliacus
• Pectineus (also sometimes by the obturator nerve)
• Sartorius
• Quadriceps femoris group:
  • Rectus femoris
  • Vastus lateralis
  • Vastus medialis
  • Vastus intermedius

Obturator Nerve (L2–L4)

• Obturator externus
• Adductor longus
• Adductor brevis
• Adductor magnus (adductor portion)
• Gracilis
• Pectineus (occasionally – shared with femoral nerve)

Lumbosacral Trunk (L4–L5)

• No direct motor innervation
  • Contributes to sacral plexus (sciatic nerve, etc.)

Sacral plexus

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